The present invention relates to the electrical, electronic, and computer arts, and, more particularly, to methods for introducing air gaps into dielectric materials with metal contacts in semiconductor devices.
Dielectric materials in semiconductor devices must be of sufficient mechanical strength to withstand the many processing steps that go into forming these devices. These steps may include, for example, lithography, deposition, wet and dry etching, and chemical mechanical polishing (CMP). At the same time, many of these dielectric layers must be able to accommodate metal features that may be tensily or compressively stressed. Without sufficient mechanical strength, a dielectric layer may simply buckle or collapse.
While introducing air gaps into dielectric layers is an effective means for decreasing the dielectric constants of these layers and reducing interconnect capacitance, the air gaps also have the undesirable effect of reducing the mechanical strengths of the dielectric layers into which they are introduced. As a result, conventional dielectric materials that contain air gaps may not be suitable for many of the dielectric features in a given integrated circuit. Dielectric layers in the middle-of-line (MOL), which, in a planar MOSFET, overlie the source and drain diffusions and contain the diffusion contacts, may be good examples. The diffusion contacts in the MOL tend to be significantly smaller than the metal interconnects in the back-end-of-line (BEOL), and tend to come in a wide range of sizes and shapes (e.g., 1×1 vias, and 1×2 and 1×4 structures). As a result, these MOL metal features may not provide a lot of mechanical stability to the MOL dielectric, and more reliance must be placed on the mechanical stability of the MOL dielectric itself. The metal features in the MOL are also often formed of tungsten deposited by chemical vapor deposition (CVD), which tends to be tensile stressed. Accordingly, dielectric materials with air gaps are typically not well suited for use as MOL dielectrics because of the mechanical weaknesses induced by the presence of the air gaps.